[dre7]

Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

[Nick Gorham]

dre7 wrote:
Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew


Not sure what "Classic" means in this context, but these come to mind.

http://www.borderpatrol.co.uk/index.htm

--
Nick

[Andre Jute]

On Oct 2, 3:24 pm, Nick Gorham wrote:
dre7 wrote:
Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

Not sure what "Classic" means in this context, but these come to mind.

http://www.borderpatrol.co.uk/index.htm

I'd go for classic: they startted in business making better power
supplies for Audio Innovations amps. In any event, I'm glad to have
the link: it has motivated me to take out my AI Classic Stereo 25 for
a bit of a play; that's definitely a classic design, if not as
fabulous (nor as expensive!) as AI's "The First" which Andy was saying
the other day is still in production by another company.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review

[dre7]

O.K. So would I be correct in assuming the reason for this is that
manufacturers such as Marantz, Fisher, McIntosh, etc. could conveniently
get power transformers wound to their exact specifications? I mean, we can
too, but for obviously more money and effort. I personally like choke input
power supplies, but all the ones I've built have been so because I've had to
bodge something out of scrounged iron.

Andrew


Bret Ludwig wrote in message
oups.com...
On Oct 2, 4:02 pm, "dre7" wrote:
Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke
input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

You mean a choke first POWER SUPPLY? No, none.

None of any size, guitar or hi-fi. Some early Hammond amps, in
Hammond tone cabs I think.

RCA used a half-choke-input supply on the famous design using
regulated screen operation of two 7027s in lots of RC manuals and the
HF-110 pamphlet. Not a produced design. But a virtuoso effort that
beat Mc numbers (but probably at higher AC power and tube consumption)
with off the shelf parts.

[Phread]

"Bret Ludwig" wrote in message oups.com...
On Oct 2, 4:02 pm, "dre7" wrote:
Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

You mean a choke first POWER SUPPLY? No, none.

None of any size, guitar or hi-fi. Some early Hammond amps, in
Hammond tone cabs I think.

RCA used a half-choke-input supply on the famous design using
regulated screen operation of two 7027s in lots of RC manuals and the
HF-110 pamphlet. Not a produced design. But a virtuoso effort that
beat Mc numbers (but probably at higher AC power and tube consumption)
with off the shelf parts.

What do you mean by "half choke input", Bret? The RCA supply has an 8 henry
choke after the rectifier followed by 20uf feeding the output tranny.

Fred

[Patrick Turner]

dre7 wrote:

Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

I have never ever seen a choke input used in any **domestic** hi-fi amp,
regardless of its class.

It would suit a class B SS amp, and I have used choke inputs in modified
SS amps, eg,

http://www.turneraudio.com.au/solidstateamps2bjts.html

The choke input PSU used in this massive amp had no better regulation
than the original
configuration. But it did bring some benefits.

Recently while perfoming singing lesson operations on a tubed CR Audio
Developments
Woodham, I placed a choke input into the PSU to drop the stupid 650v B+
rail
from a Cap input PSU down to a sensible and reliable +430V.

Most ppl have no clue as to how to build a decent choke input set up,
and bean counters always say no to the cost of the choke.

The choke input really has a benefit in class B or class C amps where
PSU drain
varies a lot, and where one wants to keep peak charge currents flowing
into capacitors
low. This is the case in class B tube amps where tube rectifiers are
used.

One finds that with silicon rectifiers and large caps without a choke,
the B+ regulation can be better than with any
choke input. Hence choke inputs are not used.

A choke input must have critical L with reactance = RL at the lowest
current, or bleed current,
so you need a lot of L. This means lots of wire and turns, and the B+
regulation always suffers from the
dc voltage drop across the winding resistance at least as well as across
winding resistances.
So a 100 ohm dcr choke will have a Vdc drop of 20V with a change in Ia
of 200mA only, and regulation is no
better than silicon and caps only.
A rule of thumb is that for where one wants really good choke input
regulation, ie,
with negligible Vdc drop caused by winding resistance, one needs to have
a choke the same weight
as the power transformer for the amp.

Charge currents in cap inputs can be reduced a lot by simply
adding series R between rectifiers and the reservoir cap until one
reduces the B+ voltage about 5%
from what it is without such R. The number of degrees if the charge
cycle are increased, and the peak chage reduced.
The amount of resistance can easily be worked out experimentally, and a
balance between peak currents
and B+ reg worked out. Hi-fi tube amps mostly don't need well regulated
PS because they
spend 98% of their lives working in class A with an unchanging I demand
on the psu.

OR one uses a low value cap as a reservoir, then a choke, then any
larger sized C for good filtering.
C1 should have reactance at least 1/20 of dc RL, the filter choke should
have reactance at least 10 times the reactance
of C1, and C2 have reactance at least 1/10 of filter choke reactance.
Regardless of what chokes and caps one uses, the resonance of the last
LC section should not be above 7Hz.

The added R can reduce peak charge I from say 10 times Idc to around 3 x
Idc,
and capacitors suffer less, because they do have finite ripple I
ratings.
Usually, the reduction of I peaks reduces the transformer noise
considerably.
The heat dissipated in the HT winding become low enough, since it
depends on I squared.
Better a hot resistance rather than a hot winding.
And in the case of a fault, the resistance fries and goes open before
anything else.

Patrick Turner.

[Andre Jute]

On Oct 3, 3:57 am, Patrick Turner wrote:

A rule of thumb is that for where one wants really good choke input
regulation, ie,
with negligible Vdc drop caused by winding resistance, one needs to have
a choke the same weight
as the power transformer for the amp.

Standard procedure in topgrade Class A1 SET amps is to use two choke
input sections, LCLC, because of the mathematical multiplier of the
second choke working on a reduced ripple. However, just as Patrick
says, you end up with the weight of the power transformer in chokes
(or more). And, of course, two chokes always drop slightly more
voltage across the copper than one big one, so it isn't a cheap
procedure in any sense of the word.

Of course with twin chokes, or separately brought out ends to double
windings on the same core, you may have other opportunities and
advantages; see:
"Linking double coil chokes for noise reduction" at
http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm
and "Phasing double coil chokes" at
http://members.lycos.co.uk/fiultra/K...hn%20Byrns.htm

Of course, the doublewound is never likely to be seen on a commercial
amp, which is what the OP wants, so I have made a separate subthread.

Do you sincerely want the best sound?
is a question quite as loaded as
Do you sincerely want to be rich?

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review

On Oct 3, 3:57 am, Patrick Turner wrote:
dre7 wrote:

Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

I have never ever seen a choke input used in any **domestic** hi-fi amp,
regardless of its class.

It would suit a class B SS amp, and I have used choke inputs in modified
SS amps, eg,

http://www.turneraudio.com.au/solidstateamps2bjts.html

The choke input PSU used in this massive amp had no better regulation
than the original
configuration. But it did bring some benefits.

Recently while perfoming singing lesson operations on a tubed CR Audio
Developments
Woodham, I placed a choke input into the PSU to drop the stupid 650v B+
rail
from a Cap input PSU down to a sensible and reliable +430V.

Most ppl have no clue as to how to build a decent choke input set up,
and bean counters always say no to the cost of the choke.

The choke input really has a benefit in class B or class C amps where
PSU drain
varies a lot, and where one wants to keep peak charge currents flowing
into capacitors
low. This is the case in class B tube amps where tube rectifiers are
used.

One finds that with silicon rectifiers and large caps without a choke,
the B+ regulation can be better than with any
choke input. Hence choke inputs are not used.

A choke input must have critical L with reactance = RL at the lowest
current, or bleed current,
so you need a lot of L. This means lots of wire and turns, and the B+
regulation always suffers from the
dc voltage drop across the winding resistance at least as well as across
winding resistances.
So a 100 ohm dcr choke will have a Vdc drop of 20V with a change in Ia
of 200mA only, and regulation is no
better than silicon and caps only.
A rule of thumb is that for where one wants really good choke input
regulation, ie,
with negligible Vdc drop caused by winding resistance, one needs to have
a choke the same weight
as the power transformer for the amp.

Charge currents in cap inputs can be reduced a lot by simply
adding series R between rectifiers and the reservoir cap until one
reduces the B+ voltage about 5%
from what it is without such R. The number of degrees if the charge
cycle are increased, and the peak chage reduced.
The amount of resistance can easily be worked out experimentally, and a
balance between peak currents
and B+ reg worked out. Hi-fi tube amps mostly don't need well regulated
PS because they
spend 98% of their lives working in class A with an unchanging I demand
on the psu.

OR one uses a low value cap as a reservoir, then a choke, then any
larger sized C for good filtering.
C1 should have reactance at least 1/20 of dc RL, the filter choke should
have reactance at least 10 times the reactance
of C1, and C2 have reactance at least 1/10 of filter choke reactance.
Regardless of what chokes and caps one uses, the resonance of the last
LC section should not be above 7Hz.

The added R can reduce peak charge I from say 10 times Idc to around 3 x
Idc,
and capacitors suffer less, because they do have finite ripple I
ratings.
Usually, the reduction of I peaks reduces the transformer noise
considerably.
The heat dissipated in the HT winding become low enough, since it
depends on I squared.
Better a hot resistance rather than a hot winding.
And in the case of a fault, the resistance fries and goes open before
anything else.

Patrick Turner.

[Patrick Turner]

Bret Ludwig wrote:

Using one choke as a first element is almost unheard of. Usually you
have two chokes, a "swinging" and a "smoothing" choke. These are two
different parts. Their combined weight and cost is usually more than
the power transformer.

The swinging choke and smoothing choke indeed make a nice PS usually
for clean supply to HV gear, such as a transmitter or class AB modulator
using tubes like the
813, 211, 845, 1606 etc.

The caps don't need to be huge C value, and can be sealed 20uF paper in
oil rated for
1,000V or more. The LCLC arrangement is then essential for a smoothed
supply
where we are using 866 rectifiers and low value C.

Such gear has never ever been bought by home users of hi-fi gear.
Many radio amateurs would be familiar with such choke input techniques.


Best source of information for small apps (under 1 kW) is the ARRL
Radio Amateur's Handbook from the AM ham era. Overseas resources like
RSGB, often superior to ARRL or W6SAI on some subjects, do not cover
this too well because high power AM was mostly exclusive to wealthy
hams who bought factory rigs outside the US.

A friend of mine here bought an 800W capable AM transmitter as ex-army
surplus gear
in about 1962, and he still has it. It has 813 PP output, and lots of
807s within, plus
LCLC type PSU.

As a thouroughly illegal ham using this he found he could reach around
the world
using his shed roof as the antenna. He posed as VK211, which was the
callsign of a guy who'd died
just before be began his pirating which went on for years without anyone
detecting anything,
although the PMG did get supsicious.
The PMG ( commumications authority ) once went searching for the origin
of the this high
power signal but he watched with amusement as the direction finding van
drove right past his house,
unable to indicate direction, because the signal totally overloaded the
gear they had.
He worked for the PMG, and knew what their capabilities were....


All AM ham transmitters over a few watts, or almost all, used high
level plate modulation. An audio amplifier, standard in all respects
but one, of approximately half the key-down CW power of the
transmitter was used. The change was the output transformer which was
an air gapped unit like a SE unit because the plate current of the
transmitter flowed through its primary.

And maybe saturable reacatnces were used... all rather quaint....

Patrick Turner.

[E M F]

"Phread" wrote in message
...

"Bret Ludwig" wrote in message
oups.com...
On Oct 2, 4:02 pm, "dre7" wrote:
Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke
input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

The Altec Lansing 256 B theatre amp used a choke input supply with 4 VR
tubes
to regulate the 807 screens and the low level stages..............class
AB/2.....GC

[BFoelsch]

On Tue, 2 Oct 2007 14:02:16 -0700, "dre7" wrote:

Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

McIntosh 50W & 50W-2

[dre7]

Thanks! Really interesting design. It appears to use an interstage as
well. I'm looking at a schematic on eBay that's pretty hard to read. It
looks like there's a cap across the choke. Did they use a resonant choke?

Andrew

BFoelsch wrote in message
...
On Tue, 2 Oct 2007 14:02:16 -0700, "dre7" wrote:

Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke
input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

McIntosh 50W & 50W-2

[Patrick Turner]

dre7 wrote:

Thanks! Really interesting design. It appears to use an interstage as
well. I'm looking at a schematic on eBay that's pretty hard to read. It
looks like there's a cap across the choke. Did they use a resonant choke?

Many old designs of choke inputs featured a C+R zobel network across the
choke
to give the choke a damping resistance load at HF.
The C size wasn't ever high enough to give an LC parallel resonance at
hum F, or twice mains F.

But in my experience there is much more to be gained if the C is given a
value
so ZC = ZL when the L is at its maximum value, which is when Idc is at
it's minimum, ie
when the tubes are conducting minimal dc at idle and set up for class B
with most
of the current being bleeder current.

But L is max when idle condition Idc in the minimum.

Today, we'd never bother with building a class B amp, and so the only
time
Idc will be low and when we want to stop the B+ soaring is when there
are no output tubes plugged in and we want to
test the PSU.
Suppose you had an amp load of 400V at 250mA at idle, typical for class
AB1, with 4 x EL34 etc.
Without output tubes, Idc might be only 25mA.

So maximum RL at lowest Idc = 400 / 0.025 = 16,000 ohms.

Minimum Critical L to ensure soaring B+ does not happen for 50Hz mains
16,000 / 900
= 17.8H.

This choke must be able to cope with 250mAdc without dc or ac
saturation, or without getting hot
due to dropping too much Vdc across itself. The low Vdc across the choke
means Rw is low,
so the choke has a large core to accomodate the turns, and the low Rw
means good regulation
of B+ despite change to idc from 25mA to 250 mA.

One can much reduce whatever ripple voltage at 100Hz appears at the cap
after the choke
by adding a series R&C network across the choke where the C resonates
with the 17.8H, but where the R value is adjusted to give minimum 200Hz
content at the cap.

An almost "free" reduction of 100Hz hum at the cap of 15dB below what
happens with no R&C.
Not only that, the Idc required to hold down soaring can be much less
than the
25mA; more like 10mA, allowing less power waste in a bleeder.

The other possibility is where we cannot hace a choke which is full
value required for minimum Idc.
The cap plus R then will allow about a 50% reduction in initial L value.
In this example, say we had a gapped choke which had only 9H at 25mAdc.
The cap value in uF required for resonance at 100Hz = 25.35 x 1,000,000
/ ( F squared x L in mH )
= 0.281uF. R must be trialed and adjusted in the amp.
The C value isn't very critical and somewhere above 0.27uF, a standard
value should be used.
Polyester and 630V rated is OK.

R could be 100ohms, 5W, wire wound.

So you should find this will save having to make the larger 18H choke,
yet give just as good performance.

Choke inductance falls as the Idc rises, since the Idc changes the µ of
the iron.
At 250mA, iron µ will have reduced to about halve the L, but it does not
matter,
Vdc will remain fairly unchanged if Si diodes are used and Rw of the
tranny and choke is low.

Let us suppose in this case we did have 18H max, and the C after the L =
470uF.
ZL = 18H x 628 = 11,300 ohms at Idc min = 25mA, and ZC = 3.4ohms, at
100Hz.

The attenutation factor = ZC / ZL = 3.4 / 11,300 = 0.0003.

There will be approx 215Vrms of 100Hz at the ac input to the choke drom
teh diodes.

The LC reduces this to 0.0003 x 215Vrms = 0.064 Vrms.

With only 9H, the Vripple would be 0.128Vrms, and B+ will have begun to
soar and be about +480V.
With the resonant C&R in place, expect Vr = 0.025Vrms, and B+ will be
about +430V.

If it was possible to maintain the L value at the same value between
25mA and 250mA,
you'd find the Vripple would remain constant, because the C will remain
constant
and same 215Vrms appears before the choke.

But usually L about halves with 10 x Idc min, so in fact Vr will about
double
But its a good result, what I have outlined above.

As long as ZL 10 x ZC, the simple calculation of attenutation factor
will be correct enough.

Patrick Turner.



Andrew

BFoelsch wrote in message
...
On Tue, 2 Oct 2007 14:02:16 -0700, "dre7" wrote:

Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke
input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

McIntosh 50W & 50W-2

[Multi-grid]

On Oct 3, 12:02 am, "dre7" wrote:
Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

hey-Hey!!!,
Sure! One would be the Altec 1570. Full bridge of 5R4's feeding the
Peerless 17266 choke, terminated with a small oil cap 8? uF.
cheers,
Douglas

[dre7]

Douglas and Patrick:

Thanks both for the information!

Andrew


Multi-grid wrote in message
oups.com...
On Oct 3, 12:02 am, "dre7" wrote:
Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke
input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

hey-Hey!!!,
Sure! One would be the Altec 1570. Full bridge of 5R4's feeding the
Peerless 17266 choke, terminated with a small oil cap 8? uF.
cheers,
Douglas

[Multi-grid]

On Oct 25, 4:17 am, "dre7" wrote:
Douglas and Patrick:

Thanks both for the information!

Andrew

Multi-grid wrote in message

oups.com...



On Oct 3, 12:02 am, "dre7" wrote:
Hey RATs:

Can anyone recall a "classic" commercially made amp that used a choke
input?
Not a guitar amp, but hifi. Been sifting through schematics but haven't
happened upon one yet.

Andrew

hey-Hey!!!,
Sure! One would be the Altec 1570. Full bridge of 5R4's feeding the
Peerless 17266 choke, terminated with a small oil cap 8? uF.
cheers,
Douglas- Hide quoted text -

- Show quoted text -

I think some of the other IPC/Altec amps did it too. Fuzzy memory and
all that, but 1520 and 1530 may also have LC filters.
cheers,
Douglas